1. Field of the Invention
The present invention generally relates to solid state image pickup apparatus and, more particularly, to a CCD (charge coupled device) defective pixel correcting circuit for use in a video camera or the like, for example.
2. Description of the Prior Art
In conventional video cameras using a solid state image pickup device such as a CCD or the like, there is utilized a defective pixel correcting circuit for correcting a so-called defective pixel that is one of the pixels of the CCD. The defective pixel is produced when or after the CCD is manufactured and outputs a signal of a characteristic level.
Further, there is known a video camera in which an analog output signal from a CCD is converted into a digital signal and then a desired processing is effected in a digital fashion. In such a video camera which carries out the digital signal processing, a defective pixel correcting circuit detects a defective pixel of the CCD in advance, and stores position data and various data relating to the defective pixel in a ROM (read only memory) or the like. Then, when the video camera is in use, the analog signal from the defective pixel is supplied with the video signal from the other pixels of the CCD through a low-pass filter to an analog-to-digital converter and then the pixel data from the defective pixel is replaced by means of video data output from a pixel near the defective pixel. Therefore, in the video signal obtained from the CCD, the pixel data output from a defective pixel which generates a signal of a particular level can be substantially corrected and a satisfactory reproduced image can be obtained. The above technique has been proposed by the assignee of the present application and is specifically disclosed in U.S. Pat. No. 5,144,446.
In the above CCD defective pixel correcting circuit, the analog output from the CCD is supplied through a low-pass filter and converted into a digital signal by the analog-to-digital converter. Then, of the digital signals from the analog-to-digital converter, the digital signal identified by the position data representative of the defective pixel is used in interpolating a digital signal to replace the output of the defective pixel. The reason that the analog output from the CCD is processed by the low-pass filter prior to the processing by the analog-to-digital converter is to prevent a false signal from being produced when the sampling is carried out in the analog-to-digital converter.
FIGS. 1A and 1B show the condition that the output from the defective pixel in the CCD is filtered by the low-pass filter.
When the output from the defective pixel as shown in FIG. 1A is filtered by the low-pass filter, a waveform of the signal is widened by the characteristic of the low-pass filter as shown in FIG. 1B. Accordingly, when the signal shown in FIG. 1B is converted by the analog-to-digital converter, there is generated a signal which is different from the defective pixel output (see FIG. 1A) of the CCD. If this signal is interpolated, then the signal having the widened waveform is output as it is because this signal is interpolated in response to the output of the defective pixel shown in FIG. 1A.
Therefore, there is proposed an interpolation technique prior to the analog to digital conversion, and in which the defective pixel data is replaced with preceding pixel data by holding pixel data corresponding to the position of a defective pixel of a CCD output by a sample and hold circuit. However, according to such interpolation technique, as shown in FIG. 2, when an image pattern is displayed with a frequency increased, particularly, in the horizontal direction, the correction of a defective pixel output by replacing the same with the preceding pixel output, as at h, has the disadvantage that a quality of a displayed image is deteriorated considerably.
Further, when a defective pixel is corrected by interpolation using pixel data near the defective pixel output in the CCD output, a time lag occurs between a signal processing system for processing the CCD output and an interpolation system for interpolating the defective pixel. There is then the possibility that a defective pixel cannot be corrected accurately.